The method for radioiodine testing of functioning thyroid carcinoma was studied. The results show that performance of the whole-body scan two weeks after stopping T3 provides a sufficient stimulus to detect a functioning thyroid remnant or metastasis. A study of serum TSH during the development of hypothyroidism in these patients showed a transient, marked increase of the alpha-subunit followed by increase in TSH-beta and complete TSH. In some cases with the syndrome "inappropriate TSH secretion", there is inhibition of metabolism of thyroxine to reverse triiodothyronine. This appears to be a sensitive index of peripheral resistance to thyroid hormone. A new syndrome of autoimmune thyroiditis with transient hypothyroidism has been described. This occurs postpartum and evolves during a period of up to one year following delivery.